feat(installer): use go-git to clone

Integrate `go-git` into the application, and use it to `git clone`
cheatsheets when the installer runs.

Previously, the installer required that `git` be installed on the system
`PATH`, so this change has to big advantages:

1. It removes that system dependency on `git`
2. It paves the way for implementing the `--update` command

Additionally, `cheat` now performs a `--depth=1` clone when installing
cheatsheets, which should at least somewhat improve installation times
(especially on slow network connections).

vendor/github.com/ProtonMail/go-crypto/openpgp/internal/ecc/ed25519.go

@@ -0,0 +1,111 @@
+// Package ecc implements a generic interface for ECDH, ECDSA, and EdDSA.
+package ecc
+
+import (
+	"crypto/subtle"
+	"io"
+
+	"github.com/ProtonMail/go-crypto/openpgp/errors"
+	ed25519lib "github.com/cloudflare/circl/sign/ed25519"
+)
+
+const ed25519Size = 32
+type ed25519 struct {}
+
+func NewEd25519() *ed25519 {
+	return &ed25519{}
+}
+
+func (c *ed25519) GetCurveName() string {
+	return "ed25519"
+}
+
+// MarshalBytePoint encodes the public point from native format, adding the prefix.
+// See https://datatracker.ietf.org/doc/html/draft-ietf-openpgp-crypto-refresh-06#section-5.5.5.5
+func (c *ed25519) MarshalBytePoint(x []byte) []byte {
+	return append([]byte{0x40}, x...)
+}
+
+// UnmarshalBytePoint decodes a point from prefixed format to native.
+// See https://datatracker.ietf.org/doc/html/draft-ietf-openpgp-crypto-refresh-06#section-5.5.5.5
+func (c *ed25519) UnmarshalBytePoint(point []byte) (x []byte) {
+	if len(point) != ed25519lib.PublicKeySize + 1 {
+		return nil
+	}
+
+	// Return unprefixed
+	return point[1:]
+}
+
+// MarshalByteSecret encodes a scalar in native format.
+// See https://datatracker.ietf.org/doc/html/draft-ietf-openpgp-crypto-refresh-06#section-5.5.5.5
+func (c *ed25519) MarshalByteSecret(d []byte) []byte {
+	return d
+}
+
+// UnmarshalByteSecret decodes a scalar in native format and re-adds the stripped leading zeroes
+// See https://datatracker.ietf.org/doc/html/draft-ietf-openpgp-crypto-refresh-06#section-5.5.5.5
+func (c *ed25519) UnmarshalByteSecret(s []byte) (d []byte) {
+	if len(s) > ed25519lib.SeedSize {
+		return nil
+	}
+
+	// Handle stripped leading zeroes
+	d = make([]byte, ed25519lib.SeedSize)
+	copy(d[ed25519lib.SeedSize - len(s):], s)
+	return
+}
+
+// MarshalSignature splits a signature in R and S.
+// See https://datatracker.ietf.org/doc/html/draft-ietf-openpgp-crypto-refresh-06#section-5.2.3.3.1
+func (c *ed25519) MarshalSignature(sig []byte) (r, s []byte) {
+	return sig[:ed25519Size], sig[ed25519Size:]
+}
+
+// UnmarshalSignature decodes R and S in the native format, re-adding the stripped leading zeroes
+// See https://datatracker.ietf.org/doc/html/draft-ietf-openpgp-crypto-refresh-06#section-5.2.3.3.1
+func (c *ed25519) UnmarshalSignature(r, s []byte) (sig []byte) {
+	// Check size
+	if len(r) > 32 || len(s) > 32 {
+		return nil
+	}
+
+	sig = make([]byte, ed25519lib.SignatureSize)
+
+	// Handle stripped leading zeroes
+	copy(sig[ed25519Size-len(r):ed25519Size], r)
+	copy(sig[ed25519lib.SignatureSize-len(s):], s)
+	return sig
+}
+
+func (c *ed25519) GenerateEdDSA(rand io.Reader) (pub, priv []byte, err error) {
+	pk, sk, err := ed25519lib.GenerateKey(rand)
+
+	if err != nil {
+		return nil, nil, err
+	}
+
+	return pk, sk[:ed25519lib.SeedSize], nil
+}
+
+func getEd25519Sk(publicKey, privateKey []byte) ed25519lib.PrivateKey {
+	return append(privateKey, publicKey...)
+}
+
+func (c *ed25519) Sign(publicKey, privateKey, message []byte) (sig []byte, err error) {
+	sig = ed25519lib.Sign(getEd25519Sk(publicKey, privateKey), message)
+	return sig, nil
+}
+
+func (c *ed25519) Verify(publicKey, message, sig []byte) bool {
+	return ed25519lib.Verify(publicKey, message, sig)
+}
+
+func (c *ed25519) ValidateEdDSA(publicKey, privateKey []byte) (err error) {
+	priv := getEd25519Sk(publicKey, privateKey)
+	expectedPriv := ed25519lib.NewKeyFromSeed(priv.Seed())
+	if subtle.ConstantTimeCompare(priv, expectedPriv) == 0 {
+		return errors.KeyInvalidError("ecc: invalid ed25519 secret")
+	}
+	return nil
+}